1. Field of the Invention
This invention relates to semiconductor materials having enhanced thermoelectric properties and preparation of such materials.
2. Related Art
Thermoelectric generators convert heat energy directly into electrical energy without moving parts. They are reliable, operate unattended in hostile environments and are also environmentally friendly. The basic theory and operation of thermoelectric devices has been developed for many years. Such devices may be used for heating, cooling, temperature control, power generation and temperature sensing. Modern thermoelectric coolers typically include an array of thermocouples which operate by using the Peltier effect.
Thermoelectric devices are coolers, heat pumps, and power generators which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other device used to transfer heat energy. The principal difference is that thermoelectric devices function with solid state electrical components (thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of the materials used in fabrication of the thermoelectric device.
The dimensionless figure of merit ZT represents the coupling between electrical and thermal effects in a material and is defined as:
xe2x80x83ZT=S2"sgr"T/xcexaxe2x80x83xe2x80x83(1)
where S, "sgr", xcexa, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The basic thermoelectric effects are the Seebeck and Peltier effects. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The complementary effect, the Peltier effect, is the phenomenon used in thermoelectric refrigeration and is related to heat absorption accompanying the passage of current through the junction of two dissimilar materials.
ZT may also be stated by the equation:                     ZT        =                                            s              2                        ⁢                          xe2x80x83                        ⁢            T                                ρ            ⁢                          xe2x80x83                        ⁢            κ                                              (        2        )            
xcfx81=electrical resistivity
"sgr"=electrical conductivity
electrical       electrical    ⁢          xe2x80x83        ⁢    conductivity    =      1          electrical      ⁢              xe2x80x83            ⁢      resistivity      
xe2x80x83or "sgr"=1/xcfx81
Thermoelectric materials such as alloys of Bi2Te3, PbTe and BiSb were developed thirty to forty years ago. Semiconductor alloys such as SiGe have also been used in the fabrication of thermoelectric devices. Commercially available thermoelectric materials are somewhat expensive. In addition, they are generally limited to use in a temperature range between 200K and 1300K with a maximum ZT value of approximately one. The efficiency of the thermoelectric devices using these materials remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of 200 to 300K, maximum ZT of current state of the art thermoelectric materials remains limited to values of approximately 1, except for Texe2x80x94Agxe2x80x94Gexe2x80x94Sb alloys (TAGS) which may achieve a ZT of 1.2 in a very narrow temperature range. Thermoelectric materials such as Si80Ge20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have a ZT approximately equal to 0.7 from 500 to 1300K.
However, for many applications with heat source temperature ranges between 100C and about 350C, there exists a gap between the low temperature state-of-the-art thermoelectric materials (Bi2Te3-based alloys) and the intermediate temperature materials (PbTe-based alloys) and TAGS (Texe2x80x94Agxe2x80x94Gexe2x80x94Sb). Consequently, the applications of current thermoelectric materials are limited because of the relatively low efficiency of the thermoelectric materials as well as their relatively high cost.
Therefore, what is needed are more efficient new thermoelectric materials. In addition, what is needed are inexpensive thermoelectric materials. What is further needed are new thermoelectric materials with an expanded range of applications.
Whatever the merits of the prior techniques and methods, they do not achieve the benefits of the present invention.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding this specification, the present invention discloses new high performance p-type thermoelectric materials having enhanced thermoelectric properties and the methods of preparing such materials.
In accordance with one aspect of the present invention, p-type semiconductor materials are formed from alloys of Zn4Sb3 for use in manufacturing thermoelectric devices with substantially enhanced operating characteristics and improved efficiency as compared to previous thermoelectric devices.
Two methods of preparing p-type Zn4Sb3 are described below, and include a crystal growth method and a powder metallurgy method. One crystal growth method for p-type single crystals is a modified Bridgman gradient-freeze technique. Namely a Bridgman Two Zone furnace and a sealed container have been modified for use in preparation of semiconductor materials in accordance with the present invention. A gradient freeze technique can be used in accordance with the present invention to produce a crystal of xcex2-Zn4Sb3 having a hexagonal rhombohedral crystal structure.
One powder metallurgy method is a hot-pressing method which includes preparing the Zn4Sb3 compound as polycrystalline samples by direct reaction of elemental powders of Zn and Sb and subsequent hot-pressing. The use of a hot-pressing method in accordance with the present invention produces large, polycrystalline ingots of semiconductor alloys. An isothermal furnace and a sealed container have been modified for use in preparation of semiconductor alloys in accordance with the present invention.
The present invention allows the use of high ZT materials in the manufacture of high efficiency thermoelectric energy conversion devices such as electrical power generators, heaters, coolers, thermocouples and temperature sensors. By using semiconductor alloys to form thermoelectric devices, such as p-type Zn4Sb3 and related alloys which have been prepared in accordance with the present invention, the overall efficiency of the thermoelectric device is substantially enhanced. For example, thermoelectric elements fabricated from semiconductor materials such as Zn4Sb3 have figures of merit ZT of about 1.4 at a temperature of about 350C.
A further important technical advantage includes the use of semiconductor materials prepared in accordance with the present invention in the manufacture of a xe2x80x9cPowerstickxe2x80x9d power source. Other thermoelectric devices manufactured from semiconductor materials fabricated in accordance with the present invention may be used in waste heat recovery systems, automobiles, remote power generators, temperature sensors and coolers for advanced electronic components such as field effect transistors.
A feature of the present invention is the ability to obtain increased efficiency from a thermoelectric device by using semiconductor materials and desired thermoelectric properties in fabrication of the thermoelectric device. Another feature of the present invention is to have a relatively high thermoelectric figure of merit for a p-type material between 200C and 350C. A further feature of the present invention is that the compound Zn4Sb3 has a complex crystal structure which results in exceptionally low thermal conductivity values which is highly desirable to obtain good thermoelectric properties.
An advantage of the present invention is that the range of applications of thermoelectric generators is expanded. Another advantage is that the thermoelectric materials of the present invention are substantially cheaper than current state-of-the-art thermoelectric materials (such as Bi2Te3-based alloys, PbTe-based alloys, and TAGS (Texe2x80x94Agxe2x80x94Gexe2x80x94Sbxe2x80x94)) and are especially viable for applications where cost is critical. A further advantage of the present invention is that higher ZT values can be achieved with additional optimization of the compounds (changing doping levels) and also by forming solid solutions with isostructural compounds, such as Cd4Sb3. For instance, solid solutions of the present invention can consist of Zn4xe2x88x92xAxSb3xe2x88x92yBy wherein 0xe2x89xa6xxe2x89xa64 and wherein A is a transition metal, B is a pnicogen, and 0xe2x89xa6yxe2x89xa63. In addition, the materials of the present invention can be used in more efficient thermoelectric generators and also for waste heat recovery and automobile industry applications, for example.